World War II, also called Second World War, conflict that involved virtually every part of the world during the years 1939–45. The principal belligerents were the Axis powers—Germany, Italy, and Japan—and the Allies—France, Great Britain, the United States, the Soviet Union, and, to a lesser extent, China. The war was in many respects a continuation, after an uneasy 20-year hiatus, of the disputes left unsettled by World War I. The 40,000,000–50,000,000 deaths incurred in World War II make it the bloodiest conflict, as well as the largest war, in history.

In the Battle of Stalingrad (1942–43), the advancing Germans were finally stopped by the Red …

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Along with World War I, World War II was one of the great watersheds of 20th-century geopolitical history. It resulted in the extension of the Soviet Union’s power to nations of eastern Europe, enabled a communist movement to eventually achieve power in China, and marked the decisive shift of power in the world away from the states of western Europe and toward the United States and the Soviet Union.

A gigantic mushroom cloud rising above Hiroshima, Japan, on August 6, 1945, after a U.S. aircraft …

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Axis initiative and Allied reaction

The outbreak of war

By the early part of 1939 the German dictator Adolf Hitler had become determined to invade and occupy Poland. Poland, for its part, had guarantees of French and British military support should it be attacked by Germany. Hitler intended to invade Poland anyway, but first he had to neutralize the possibility that the Soviet Union would resist the invasion of its western neighbour. Secret negotiations led on August 23–24 to the signing of the German-Soviet Nonaggression Pact in Moscow. In a secret protocol of this pact, the Germans and the Soviets agreed that Poland should be divided between them, with the western third of the country going to Germany and the eastern two-thirds being taken over by the U.S.S.R.

Having achieved this cynical agreement, the other provisions of which stupefied Europe even without divulgence of the secret protocol, Hitler thought that Germany could attack Poland with no danger of Soviet or British intervention and gave orders for the invasion to start on August 26. News of the signing, on August 25, of a formal treaty of mutual assistance between Great Britain and Poland (to supersede a previous though temporary agreement) caused him to postpone the start of hostilities for a few days. He was still determined, however, to ignore the diplomatic efforts of the western powers to restrain him. Finally, at 12:40 pm on August 31, 1939, Hitler ordered hostilities against Poland to start at 4:45 the next morning. The invasion began as ordered. In response, Great Britain and France declared war on Germany on September 3, at 11:00 am and at 5:00 pm, respectively. World War II had begun.

Overview of the German invasion of Poland (1939), which marked the beginning of World War II.

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World War II started in 1939. By the time it ended in 1945, the war involved nearly every part of the world. The two sides that fought the war were called the Axis powers and the Allies. Germany, Italy, and Japan were the major Axis powers. The major Allies were the United States, the Soviet Union, the United Kingdom (Great Britain), and France. China also aided the Allies.

Some 20 years after the end of World War I, lingering disputes erupted in an even larger and bloodier conflict-World War II. The war began in Europe in 1939, but by its end in 1945 it had involved nearly every part of the world. The opposing sides were the Axis powers-consisting mainly of Germany, Italy, and Japan-and the Allies-primarily France, Great Britain, the United States, the Soviet Union, and, to a lesser extent, China. Estimates of the number of casualties vary widely, but by any measure the war’s human cost was enormous-35 million to 60 million deaths, with millions more wounded or left homeless. (For a chronology of events for World War II, see World War II Chronology.)

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